Generally, a portable terminal such as a cellular phone, a notebook computer, a portable digital assistant (PDA) or the like is equipped with a battery therein so that the portable terminal can be used while a user moves. Such a portable terminal requires a charger for charging the battery and the charger is connected to a general commercial power supply to supply a charging current to the battery of the portable terminal.
In order for the charger to provide the charging current to the battery of the portable terminal, the charging body constituting the charger and the battery of the portable terminal should be electrically connected.
Conventional chargers are connected to a portable terminal by wire and have connection terminals for wire connection. Therefore, when charging the battery of the portable terminal, the connection terminal of the portable terminal and the connection terminal of the charger should be connected to each other.
However, in the case of the connection terminal method described above, the terminals have different specifications and shapes depending on the devices, and thus the users may have to purchase new charging devices every time.
In order to solve this problem, a non-contact magnetic induction method, i.e., a wireless charging method has been devised. The wireless charger can be classified into a magnetic induction (MI) system (i.e., standard specification) and a magnetic resonance (MR) system (i.e., a standard specification). There may be a WPC (Wireless Power Consortium) system (i.e., a standard specification), and a PMA (Power Matrix Alliance) system (i.e., a standard specification), as two representative MI systems.
Korean Patent Publication No. 10-1169661 (published on Jul. 24, 2012) discloses a wireless power receiver. The wireless power receiver is disposed adjacent to an external charge transmitter equipped with a primary coil in a non-contact manner so that a battery is charged. The wireless power receiver includes: a main body; a secondary coil connected to the main body and a battery, and which generates an induced electromotive force by an induced magnetic field generated in the primary coil; a receiving ferrite member that is disposed in a rear surface of the secondary coil so that the induced magnetic field is well induced in the secondary coil; and a magnetic shield that is disposed on a rear surface of the receiving ferrite member and that blocks the induced magnetic field from being emitted in the battery direction, wherein the magnetic shield is made of a flexible material of a light weight in which a PET film and an amorphous tape are laminated, and a heat radiation sheet is provided on a rear surface of the magnetic shield, to radiate heat generated in the secondary coil.
The heat radiation sheet is formed of a heat radiation metal plate for diffusing heat generated in the secondary coil. Such a heat radiation metal plate plays a role of heat dissipation, but eddy current is generated in the heat radiation metal plate itself, which causes a decrease in charging efficiency due to eddy current loss.
The eddy current is generated when an alternating magnetic field generated in the secondary coil is induced in the inside of the metal plate which is a conductor by the electromagnetic induction to cause an eddy current loss, and Eddy Current Loss (Pe) can be expressed by the following equation (1):pe=ke(Bmaxtf)2  [Equation 1]
where Ke denotes a material constant, Bmax denotes the maximum magnetic flux density, t denotes thickness, and f denotes frequency.
As shown in Equation 1, the eddy current loss Pe is proportional to the square of the thickness t of the heat radiation metal plate.
Therefore, if the thickness of the heat radiation metal plate is made thin, it is possible to reduce the eddy current loss, but there is a problem that the heat radiation area is reduced and the heat radiation performance is lowered. If the thickness of the heat radiation metal plate is made thick, the heat radiation performance is improved, but the charging efficiency is lowered because the eddy current loss is caused in proportion to the square of the thickness of the heat radiation metal plate.
The currently available heat radiation metal plate is manufactured to a thickness that can be lowered to a set temperature because it needs to perform the heat radiation function required by a wireless power device. Therefore, the currently available heat radiation metal plate has a problem that an eddy current loss is generated, thereby deteriorating the charging efficiency.